Parent shaking when hit play in bge
I had a similar problem when tinkering with the BGE.  I had a rudimentary cannon and large sponge rubber shells so no one gets hurt.  The empty was used to move the shells out of the barrel .  It was too close to the barrel if I remember properly and would sort of tremble.  It was long time ago but I can go back into that project and even send it to you if you want.

Try removing the parentage, move the empty about a little, then re-parent it, see if the problem goes away...